To ensure the operability of a memory cell, including storage devices such as registers or latches, each individual device is usually tested after fabrication. The devices are tested by storing a known pattern into the stored cells of the device and subsequently reading the contents of the device. If the data input to the device is not identical to the data output from the device, then the device is discarded as having one or more failed cells. To detect hard failures, where a memory cell is always one or always zero, the device is tested with different patterns, such that each cell is tested for its ability to successfully store both logic states.
Where higher reliability is needed, more extensive testing may be performed on the devices. For example, the devices may be tested at different temperature ranges to ensure the operability of the device over a specified temperature range. Similarly, the device may be tested over a specified voltage range to determine failures. In some instances, the devices will be tested after a prescribed "burn-in" period, typically, 24 to 48 hours, wherein the device is operated during the burn-in period. If the device is likely to fail during operation, it has been found that there is a high probability that the chip will fail during the burn-in period.
Nevertheless, the operational testing does not detect all errors which may occur due to processing variations and inaccuracies. A memory device may fail because some of its cells are "weak," i.e., they may be prone to upset, wherein the stored cell may flip from the stored logic state to the complementary logic state. Upset may occur in a weak cell due to several external factors. Alpha particles emitted from the packaging materials may cause a weak memory cell to switch states. Similarly, a high energy ion may impinge the memory cell, leaving electron/hole pairs which cause the cell to upset. This phenomena is known as single event upset, or SEU. Further, a burst of gamma radiation may generate electron hole pairs which push the voltage toward the mid-rail, causing the memory cell to switch states. A memory device's immunity to gamma radiation is known as its transient dose hardness. It is also possible that as parameters change with time, for example with hot electron degradation or with metal migration, a weak cell may become a failure.
Therefore, a need has arisen in the industry for a test which will detect weak memory cells in a storage device.